Our Cortex-m55 build is failing for fft_test.cc due to strict tolerance in irfft tests. Increasing tolerance slightly similar to what we did for rfft.

Can test locally with
```make -f tensorflow/lite/micro/tools/make/Makefile -j24 test_kernel_signal_fft_test TARGET=cortex_m_corstone_300 TARGET_ARCH=cortex-m55```

BUG=[287518815](http://b/287518815)